Once the meringue holds stiff peaks, add in the lime zest, lime juice, and tequila and keep mixing on high. Switch to the paddle attachment and lower the speed of the mixer. Add the softened butter one cube at a time, mixing until fully combined between each addition. If it looks too soft or like it might curdle, don’t worry. Just keep adding the butter and mixing until it is fully incorporated, it will come together on it’s own to form a smooth buttercream. Once it comes together, if it sill looks a bit too soft, just pop it into the refrigerator for 10-15 minutes to firm up.